As states legalize marijuana or its medical use, expectant mothers are taking it up in increasing numbers — another example of the many ways in which acceptance of marijuana has outstripped scientific understanding of its effects on human health.

Often pregnant women presume that cannabis has no consequences for developing infants. But preliminary research suggests otherwise: Marijuana’s main psychoactive ingredient — tetrahydrocannabinol, or THC — can cross the placenta to reach the fetus, experts say, potentially harming brain development, cognition and birth weight. THC can also be present in breast milk.

Six-year-olds born to mothers who had smoked one joint or more daily in the first trimester showed a decreased ability to understand concepts in listening and reading, in a Pittsburgh study. At age 10, children exposed to THC in utero were more impulsive than other children and less able to focus their attention.

Most troubling, children of mothers who used marijuana heavily in the first trimester had lower scores in reading, math and spelling at age 14 than their peers.

Several studies have found changes in the brains of fetuses, 18 to 22 weeks old, linked to maternal marijuana use. In male fetuses who were exposed, for instance, researchers have noted abnormal function of the amygdala, the part of the brain that regulates emotion.

It is already well documented that the developing brains of teenagers can be altered with regular marijuana use, even eventually reducing I.Q. “It could make the difference between getting an A and getting a B.”

The American Academy of Pediatrics and the American College of Obstetricians and Gynecologists both advise against prenatal cannabis use because of its links to cognitive impairment and academic underachievement. But many state and federal agencies avoid the topic.

"Smoking cannabis “does confer, in terms of birth weight, the same risk as cigarettes,” according to Dr. Marie McCormick, a pediatrician and the chairwoman of a new report on cannabis from the National Academies of Sciences, Engineering and Medicine.

“Just because they don’t have a major birth defect or overt withdrawal symptoms doesn’t mean the baby’s neurological development is not impacted.”

Most research in this area was done when the drug was far less potent. Marijuana had 12 percent THC in 2014, while in 1995 it was just 4 percent, according to the National Institute on Drug Abuse. “All those really good earlier studies on marijuana effects aren’t telling us what we need to know now about higher concentration levels,”

Few realize that THC is stored in fat and therefore can linger in a mother’s body for weeks, if not months. It’s not known whether the fetus’s exposure is limited to the hours a woman feels high.

The American College of Obstetricians and Gynecologists advises clinicians to ask pregnant women about marijuana use and to urge them to quit. (Source: Pregnant Women Turn to Marijuana, Perhaps Harming Infants, by Catherine Saint Louis, Health, New York Times, February 2, 2017)